1. Field of the Invention
The present invention relates to a storage circuit that can keep a stored logic state even when power is off, and a storage device and a signal processing circuit that include the storage circuit. The present invention also relates to an electronic device including the storage circuit or the signal processing circuit.
2. Description of the Related Art
Signal processing circuits such as central processing units (CPUs) have a variety of configurations depending on their application and are generally provided with a variety of storage circuits such as a register and a cache memory as well as a main memory for storing data or a program.
In a storage circuit such as a register or a cache memory, data needs to be read and written at higher speed than in a main memory. Thus, in general, a flip-flop is used as a register, and a static random access memory (SRAM) or the like is used as a cache memory. That is, a volatile storage circuit in which data is erased when supply of power supply voltage is stopped is used for such a register, a cache memory, and the like.
In order to reduce consumed power, there has been suggested a method of temporarily stopping supply of power supply voltage to a signal processing circuit in a period during which data is not input and output. In that method, a nonvolatile storage circuit is located in the periphery of a volatile storage circuit such as a register or a cache memory, and data is temporarily stored in the nonvolatile storage circuit. Thus, the register, the cache memory, or the like holds data even while supply of the power supply voltage is stopped in a signal processing circuit (e.g., see Patent Document 1).
In addition, in the case where supply of the power supply voltage is stopped in a signal processing circuit for a long time, data in a volatile storage circuit may be transferred to an external storage device such as a hard disk or a flash memory before supply of the power supply voltage is stopped, in which case the data can be prevented from being erased.